Wise guy swots up about yarn (5,4)
I believe the answer is:
smart alec
'wise guy' is the definition.
(I've seen this before)
'swots up about yarn' is the wordplay.
'swots' becomes 'crams' ('cram' can be a synonym of 'swot').
'up' is a reversal indicator (in down clue: letters go upwards).
'about' means one lot of letters goes inside another.
'yarn' becomes 'tale' (synonyms).
'crams' back-to-front is 'smarc'.
'smarc' placed around 'tale' is 'SMART-ALEC'.
(Other definitions for smart alec that I've seen before include "Know-all" , "Clever Dick" , "Clever sort" , "Clever person" , "insufferable type" .)